My invention provides a unique, inexpensive, hygienic, simple, precise and safe way to provide 100% oxygen by continuous positive airway pressure (CPAP) to the non-ventilated, non-dependent lung during thoracic operations utilizing one-lung ventilation. Approximately seventy-five thousand thoracic operations are done annually in the United States utilizing one-lung anesthesia. Many thousands more are done utilizing conventional two-lung anesthesia and two-lung ventilation because practitioners wish to avoid the potential for life-threatening hypoxemia (under-oxygenation of the blood) which is associated with one-lung anesthesia.
"One-lung anesthesia" is a term which is used to describe patients who undergo thoracic surgery in which only the dependent, non-operated lung is ventilated, no matter how the anesthesia is given. In general, the anesthetic is given intravenously or by inhalation or, most commonly, by a combination of these two techniques. In fact, then, "one-lung anesthesia" might be better termed "one-lung ventilation in conjunction with anesthesia for thoracic operations".
Since only one lung is ventilated, a special technique must be used to physically separate the lungs. Most commonly, this separation is accomplished by using a double-lumen endotracheal tube. As shown in FIG. 1, this tube 1 has two separate lumens or breathing channels 2 and 3 delivering oxygen and anesthetic gas through an anesthesia circuit 4 to each of the lungs 5, a Y-connector 6 to connect the proximal ends of the channels together, and a clamp 7 for disconnecting one channel from the Y-connector. When gases and oxygen are delivered to the Y-connector, both lungs are normally ventilated with fresh gas flowing through the separate channels 2 and 3 to the respective lungs. However, if the upper leg of the Y-connector 6 is clamped, then only the lower lung 8 is ventilated. Further, when the side which is clamped is opened to room air by opening a cap (not shown) on the channel 2, the lung associated with that channel will collapse. The foregoing procedure is what is done during thoracic surgery. By doing so, one creates a compact, collapsed upper lung 9 for the surgeon to operate on. More important, the operated lung 9 is no longer expanding and collapsing since no ventilation is being delivered to that side (anesthetized patients undergoing thoracic operations are paralyzed with neuromuscular blocking drugs and cannot breathe for themselves).
Thus, during one-lung anesthesia, the lungs are separated by the anesthesiologist providing positive pressure ventilation to the dependent lower lung 8 while the surgeon operates on the small, still, non-ventilated, non-dependent upper lung 9. By definition, ventilation is the physiologic process whereby lung inhalation (or inflation) and exhalation (or deflation) occur; during lung surgery, ventilation is caused by the intermittent delivery of gas under pressure by the anesthesiologist. Ventilation serves two purposes. First, oxygen is delivered to the lung where it is taken up by the blood and carried out to the body for use in metabolic processes. Second, carbon dioxide, an end-product of the body's physiologic metabolic processes, is excreted.
When one-lung ventilation is initiated during thoracic operations, the dependent ventilated lower lung 8 still participates in the delivery of oxygen to the blood. However, the non-ventilated, non-dependent upper lung 9 no longer participates in gas exchange. Ideally, all the de-oxygenated blood returning to the right side of the heart and going to the lungs encounters alveoli or air sacs which are filled with oxygen (because they have been ventilated), thereby allowing the de-oxygenated blood to replenish its supply. In the situation of one-lung ventilation, however, the blood which passes through the non-ventilated lung remains de-oxygenated and mixes with the blood passing through the ventilated lung. Thus, there is invariably a fall in overall oxygenation of the blood as compared to situations of two-lung ventilation. The body has a number of physiologic mechanisms which tend to divert blood to the ventilated lung and away from the non-ventilated lung and thereby reduce the amount of poorly oxygenated blood which mixes with that which is well oxygenated. In addition, anesthesiologists employ a number of maneuvers to maximally oxygenate the blood which flows to the single-ventilated lung (such as using 100% oxygen); however, there is a significant percentage of patients in which undertaking one-lung ventilation will cause life-threatening hypoxemia to occur.
When hypoxemia occurs during one-lung ventilation, the obvious solution is to reinstitute ventilation to the non-dependent (upper) lung. However, this situation may cause hazardous interference with the surgical procedure. Fortunately, one can still provide oxygen under gentle pressure (low levels of CPAP) to the non-dependent lung without actually ventilating (that is, without actually inflating and deflating) that lung. Gently stenting open alveoli with oxygen under constant, non-varying pressure allows the blood flowing through this non-ventilated lung now to become well oxygenated, thereby alleviating the hypoxemia. Because the pressure is constant and unvarying, the lung is not moving (and thus not ventilating), and the surgeon is able to carry out the operation with a minimum of interference. In the situation of ventilation of the dependent lung along with such CPAP oxygenation (without ventilation) of the non-dependent lung, oxygenation of the blood is occurring via both lungs while active carbon dioxide removal is occurring via only the ventilated lung. The reason for this is that active inhalation and exhalation are needed to provide carbon dioxide removal, while it is necessary only to present oxygen to the alveoli to allow oxygenation. Fortunately, patients can tolerate a modest buildup of carbon dioxide without serious harm.
Unfortunately, even though there exists a physiological solution to the problem of hypoxemia during one-lung ventilation (namely, non-dependent lung CPAP oxygenation), there is no commercially available, fully assembled, inexpensive device for implementing the solution. While several CPAP systems have been described in the anesthesia literature, the problem with all such systems is that they are "homemade" devices which must be assembled by the practitioner from individually obtained components. Most of the devices described, such as those taught by Benumof in his Anesthesia for Thoracic Surgery, published in 1987 by W. B. Saunders Company, include a pressure-measuring gauge along with a "pop-off" regulating valve and oxygen tubing; when fully assembled, most such devices cost well over $100. In addition, they are bulky and cumbersome to use.
There are several consequences of the above problem. First, very few practitioners have taken the time, trouble and expense required to procure and assemble for themselves a device which allows CPAP oxygenation to the non-dependent lung. When these individuals encounter hypoxemia during one-lung ventilation, either they "get by" with varying degrees of under-oxygenation of the patient or they resume two-lung ventilation and thereby interfere with the operation being performed. Alternatively, many practitioners elect never to use one-lung ventilation anesthesia techniques for fear of encountering hypoxemia and not having the means (a CPAP device) to treat it properly, thereby imposing a serious risk to certain patients since it is well established that physical lung separation with one-lung ventilation is mandated for certain operations (lung abscess or bronchopleural cutaneious fistula, for example). Thus, one subset of patients is being denied a required procedure for fear of a potential complication, while another subset is being given the procedure without the means to treat that complication should it occur.
Another continuous positive airway pressure administrating device has been proposed in U.S. Pat. No. 4,249,527 (Ko et al.) which relates to a complicated system for administering CPAP to patients, such as new born infants suffering from idiopathic respiratory-distress syndrome. This system, which includes a tube attached to a source of fresh air under pressure, a hose connecting the tube with a pressure control valve assembly 142, and a nasal cannula for delivering CPAP to the patient, may be controlled by adjusting the control valve 142 to expose more or fewer of openings 188, 190 to the atmosphere thereby venting more or less carbon dioxide exhaled by the patient. Thus, CPAP is being administered to both lungs while the patient is actively ventilating. Further, the valve 142 may be completely closed off or opened to provide an unlimited continuum of pressure settings. Therefore, such a system is not suitable for use with anesthetized patients undergoing one-lung anesthesia.
U.S. Pat. No. 4,261,355 (Glazener) discloses a constant positive pressure breathing apparatus for use with patients undergoing either spontaneous respiration or mechanical ventilation. If one regulates the mass flow rate of gas from a remote reservoir into a nozzle, variable levels of constant positive airway pressure can be maintained. Thus, this apparatus has a drawback in that the only way to vary the airway pressure is to control gas flow rate. Also, Glazener's device is used with a standard endotracheal tube as opposed to a double-lumen endotracheal tube.
U.S. Pat. Nos. 4,643,183, 4,593,688 and 4,098,290 disclose various breathing related apparatuses including valve means.
U.S. Pat. No. 4,598,706 discloses an apparatus for independent ventilation of two lungs by using a positive end-expiratory pressure (PEEP) valve. Also, an article, entitled "Improved Ventilation During Thoracotomy with Selective PEEP to the Dependent Lung" by Brown, et al., published in Anesthesia and Analgesia in 1977 (Vol. 56, No. 1), discloses a device for supplying PEEP to the dependent lung during two-lung ventilation in thoracotomy patients.
U.S. Pat. Nos. 3,017,881, 3,786,809, 3,906,996, 4,244,363, 4,266,540 and 4,502,481 are of background interest with respect to the present invention.